Fix trait terminology (#39)

src/target/ext/mod.rs

@@ -101,7 +101,7 @@
 //!
 //! - (library) Define a `trait OptExt: Target { ... }` with all the optional
 //! methods:
-//! - Making `OptExt` a supertrait of `Target` enables using `Target`'s
+//! - Making `OptExt` a subtrait of `Target` enables using `Target`'s
 //! associated types.
 //!
 //! ```rust,ignore